Ink jet heads cable of effecting recording by making utilization of the piezoelectric effect of piezoelectric elements have been known in the prior art.
A typical ink jet head of such a type has a piezoelectric actuator formed by laminating together a common electrode, a piezoelectric element, and an individual electrode in sequence, and an ink flow path substrate in which is formed a pressure chamber. Mounted on one surface of the piezoelectric actuator is a vibrating plate. The vibrating plate is bonded onto an ink flow path formation substrate with an adhesive agent. And, during the emission of ink, voltage is impressed on the common electrode and on the individual electrode, as a result of which the piezoelectric element undergoes expansion and contraction. The vibrating plate restricts the expansion and contraction of the piezoelectric element. Consequently, the piezoelectric actuator undergoes bending and deformation in the lamination direction. The volume of the pressure chamber is varied by the bending and deformation of the piezoelectric actuator, and ink in the pressure chamber is shot through a nozzle.
Incidentally, piezoelectric elements are formed of insulating material. Therefore, it is undesirable for the ink jet head to be left in humid circumstances for the following reasons. A typical piezoelectric element is prepared by a film forming technique such as sputtering or the like. This, however, may give rise to a void and/or a pinhole within the piezoelectric element in the course of such film preparation. Voids are likely to be produced by the abnormal growth of a piezoelectric element caused by contaminants adhering onto an ink flow path substrate. On the other hand, pinholes are likely to be produced when a piezoelectric element is sputtered onto an ink flow path formation substrate. And, if moisture enters a void or a pinhole, this results in increased leak current when voltage is impressed between both the electrodes. Therefore, there is the possibility of occurrence of dielectric breakdown.
With a view to eliminating the above-mentioned harmful effects, Japanese Patent Kokai Gazette No. (1999)235823 proposes an ink jet head provided with a cap for enclosing a piezoelectric element which is joined onto an ink flow path formation substrate, thereby making it possible to prevent water vapor from entering the space within the cap. Consequently, entrance of water vapor into a void or a pinhole of the piezoelectric element is prevented, and it can be expected that dielectric breakdown due to moisture will be prevented from occurring.
The above-described ink jet head construction is provided with a cap capable of enclosing a piezoelectric actuator. However, it does not provide any means for absorbing moisture within the cap, therefore making it difficult to maintain the space within the cap at low humidity levels. Consequently, there is still the possibility of occurrence of dielectric breakdown in the above-described prior art ink jet head.
In order to eliminate the above-mentioned harmful effects, Japanese Patent Publication No. 2754407 proposes a piezoelectric actuator provided with a moisture absorbing part having a moisture absorbing action. This moisture absorbing part is disposed in the vicinity of a piezoelectric element. Therefore, ambient humidity around the piezoelectric element is lowered by the moisture absorbing action of the moisture absorbing part, and it can be expected that dielectric breakdown due to moisture will be prevented from occurring.
The above-described piezoelectric actuator construction includes a moisture absorbing part in the vicinity of a piezoelectric element. However, it does not provide any means for isolating the piezoelectric actuator itself from moisture-containing outside air. In addition, the foregoing patent publication fails to provide any means for coping with problems arising when the moisture absorption action of the moisture absorption part becomes deteriorated. Therefore, it is difficult for the above-described piezoelectric actuator to maintain ambient humidity around the piezoelectric element at low levels. Therefore, there is the possibility of occurrence of dielectric breakdown.
Bearing in mind the foregoing drawbacks with the prior art techniques, the present invention was made. Accordingly, an object of the present invention is to prevent dielectric breakdown due to moisture from occurring in a piezoelectric actuator by specifying relative humidity around the piezoelectric actuator, the performance of a moisture absorbent, or the performance of a cap member enclosing the piezoelectric actuator.